Finding source of tracking

Hi there,I am relatively privacy-conscious although not nearly as much as many people here. I use linux on both my computers, firefox with strict tracker blocking setting enabled, ublock origin, use a privacy friendly search engine, use posteo for my email (although i still have a gmail for a couple of things which were difficult to move over to a new email, and which I am not too concerned with in terms of privacy), use vpns sometimes, and have a privacy-based android rom on my phone (GOS) with firefox and ublock origin.

Despite this, I have seen clearly targeted ads on the instagram and reddit apps recently, which were related to my search history/browsing. For instance, instagram showed me an ad for a pair of headphones I had recently viewed on bestbuy’s website on firefox. And something similar happened with a reddit ad.

Both accounts are linked to an alias of my personal email.

My question is, how would these social media accounts know about this browsing? Would it be ip-based tracking? And regardless of why, how do I put an end to this? I do not want instagram and reddit to know about my shopping/browsing habits. Any additional tips or feedback would be appreciated.

P.S. Please do not be condescending about my rather liberal approach to privacy compared to some here with higher threat models, or propose all-or-nothing solutions in terms of privacy. I have thing set up the way I do because that is the balance of privacy/convenience I have found works in my lifestyle, although I am willing to adjust things if I can make it work in my life. Privacy is a spectrum and we all have different threat models.